1. Field of the Invention
The present invention relates to the refining of hydrocarbon feedstocks. More particularly, this invention concerns a selective separation of a heavy oil feed into polar and nonpolar fractions using a specified double solvent extraction system.
2. Description of Relevant Art
Hydrocarbon feedstocks, whether derived from natural petroleum or synthetic sources, are composed of hydrocarbon and non-hydrocarbon (e.g., heteroatom containing organic molecules) components which differ in boiling point, molecular weight and chemical structure. High boiling, high molecular weight non-hydrocarbons (e.g., asphaltenes) are known to contain a greater proportion of carbon-forming constituents (i.e., coke precursors) than lower boiling naphtha and distillate fractions. Because coke precursors form coke during thermal processing (such as is employed in a modern refinery), it is desirable to remove (or at least segregate) the non-hydrocarbon components containing the coke precursors, thereby facilitating further processing of the more valuable fractions of the feedstock. One method often utilized for this segregation is solvent deasphalting.
Deasphalting is a solvent extraction process utilizing a light hydrocarbon solvent (e.g., propane, butane, pentane or heptane) to separate heavy hydrocarbon feedstocks into a deasphalted oil and a low value residue or asphalt which contains asphaltenes. See, e.g., U.S. Pat. No. 3,132,088. Unfortunately, in the typical technique, the separation is not selective in that much of the more valuable deasphalted oil is precipitated with the residue while hydrocarbons containing coke precursors are extracted with the deasphalted oil. Thus, such a technique only partially segregates the coke precursors from the more valuable fractions and results in a significant portion of the more valuable product inherently and unavoidably remaining with the coke precursor rich residue. This is particularly so with heavy crudes and oils.
Solvent extractions and various other techniques have been proposed for preparation of Fluid Catalytic Cracking (FCC) charge stock from resids. Solvent extraction, in common with propane deasphalting, functions by selection on chemical type, rejecting from the charge stock the aromatic compounds which can crack to yield high octane components of cracked naphtha. Low temperature, liquid phase sorption on catalytically inert silica is described by Shuman et al., Oil and Gas Journal, Apr. 6, 1953, page 113. U.S. Pat. Nos. 3,565,795 and 3,567,627 describe a method of separating polar materials such as high molecular weight aromatics from petroleum distillate fractions by selective solvent extraction.
The literature discloses many variations of the solvent extraction process. Thus, U.S. Pat. No. 2,928,788 discloses solvent recovery by cooling, and U.S. Pat. No. 3,177,196 discloses solvent recovery by re-extraction. U.S. Pat. Nos. 3,492,365 and 3,186,937 describe separation of aromatic from non-aromatic hydrocarbons using a two-stage extraction zone. U.S. Pat. No. 3,205,167 discloses cooling intermediate extracts so as to obtain by phase rejection an intermediate quality heart-cut raffinate.
Other patents dealing with solvent extraction processes include U.S. Pat. No. 3,280,024, which discloses a conventional duo-solvent type extraction process for separating naphthalenic hydrocarbons from mixtures using a liquid-liquid extraction procedure, wherein the extractants are complementary polar and nonpolar solvents. U.S. Pat. No. 3,317,422 teaches successive extractions with the same solvent, and U.S. Pat. No. 3,779,896 discloses single-solvent combination extraction and deasphalting. U.S. Pat. No. 4,125,458 relates to a process of simultaneously deasphalting and extracting an asphalt-containing mineral oil with a solvent mixture of a C.sub.2 -C.sub.10 hydrocarbon and NMP, with the solvent recovered by cooling. U.S. Pat. No. 3,682,815 discloses another Duo-Sol extraction process using a split feed of polar solvent. In addition, U.S. Pat. No. 4,305,812 discloses dual solvent deasphalting by polarity gradient extraction. U.K. Pat. Pub. No. 2,081,297A also discloses a dual solvent refining process using NMP. Canadian Pat. Nos. 613,224 and 1,085,334 and U.S. Pat. Nos. 3,278,415 and 2,092,739 teach dual solvent extraction methods. See also U.S. Pat. No. 3,291,718 and 3,975,396.
It is also known from U.S. Pat. No. 4,321,216 to deoil hydrocarbon sulfonates using a mixture or solvent system comprising a polar and nonpolar component. U.S. Pat. No. 4,325,818 discloses a dual solvent process for refining oil stocks using N-methyl-2-pyrrolidone to dissolve the aromatics fraction and a highly paraffinic oil as a backwash solvent.
It is known from U.S. Pat. No. 2,937,135 to employ two substantially immiscible liquid solvents to extract polynuclear aromatic materials.
U.S. Pat. No. 4,354,928 discloses deasphalting and extracting of a petroleum oil with a solvent maintained at its critical temperature and pressure so that extraction is effected by vapor-liquid phase separation.
U.S. Pat. No. 2,273,661 discloses a method for refining heavy oil by selective solvent extraction with a mixture of solvents, one which dissolves hydrocarbons of high hydrogen-carbon ratio, correlating the proportions of solvents to the temperature at which the oil is to be treated.
Finally, U.S. Pat. No. 3,380,912 discloses a combination extraction-demetalization process for heavy oils using a second extraction zone where the asphaltene-containing material is contacted with a solvent to extract aromatics and a solvent which causes rejection of the asphaltene into the extract or heavier phase.